The latest Inc. 5000 List of the fastest-growing private companies in the United States features 40 education businesses that serve children from pre-kindergarten through 12th grade, in and out of school.

The Cambridge Institute of International Education, a Massachusetts-based firm that helps high schools and colleges increase their enrollment of international students, tops the list at $9.8 million in annual revenues and 2,683 percent growth. PresenceLearning, a California company that provides live online special education services, was second with 2,634 percent growth over three years.

To be on the list, a private company must reveal its annual revenues, so the list is a snapshot of privately held education businesses willing to provide this often closely guarded information. This year’s rankings are based on their percentage revenue growth between 2010 and 2013, with companies generating at least $100,000 in 2010 and at least $2 million in 2013.

Among the businesses that identified themselves in the “education” sector, many are for college students or adult career training. The rest apply to the pre-college set. This year, the 40 companies in that Pre-K-12 category have combined annual revenues amounting to $527 million.

While it’s natural to look at who’s #1, it’s often instructive to see which companies have appeared year in and year out on the list produced by Inc. Magazine. Repeat appearances indicate that companies have been able to sustain a momentum of fast growth on an annual basis, and those that have appeared on the list repeatedly often rank lower on the list because it’s more difficult to show large percentage jumps in revenues as a company matures than it is to show them in earlier stages of its development. Companies like Microsoft, Intuit, and Zappos! have histories of qualifying for the list many years in a row.

Seven education businesses serving the Pre-K-12 category have been on the list five or more times. (Inc. reports that 908 of the 5,000 businesses it ranked this year have appeared on the list at least five times.)

Two Pennsylvania-based companies have been on the “fastest-growing” list for eight years: Schoolwires, which provides a K-12 website and community management system, ranked 36th among education companies that serve schoolchildren, and has been in the “fastest-growing” category for eight years. So has Pediatric Therapeutic Services, which provides on-site physical and speech, occupational, and psychological therapy to children with disabilities, ranked 40th. (Last year, Christiane Crawford—the president and CEO of Schoolwires—shared with us her views about the company’s continued fast growth.)

Achieve3000, a New Jersey-based company that provides differentiated instruction, has appeared on the list seven times and is the 19th fastest growing education company serving schoolchildren. ClassBook, a New York-based business that resells print and e-books to preparatory schools nationwide, has also shown up 35 times.

View the full list here.

Education is a moderately fast-growing industry, according to Inc.’s rankings by growth rate. Between 2010 and 2013, it grew at a rate of 130 percent. (This figure includes education companies that serve college students and adults in the workplace as well.) When measured by total revenues, education is one of the lowest-ranking industries at $1.2 billion.

As a category, consumer products and services grew fastest by the growth-rate yardstick, logging a rate of 206 percent. That might be why #1-ranked Fuhu, a California company that makes the Nabi, an Android tablet for kids, has chosen to stay out of the education marketplace. Instead, it sells directly to parents, even though educational software—some of it designed with DreamWorks—is one of its product offerings. (Similarly, Disney recently released its “Imagicademy” with the input of educators, but no intention to sell to the Pre-K-12 education marketplace.)

Michele Molnar is associate editor of EdWeek Market Brief. She is also a reporter who covers industry and innovation for Education Week. Michele began working as a contributing writer for Education Week in 2012, covering parents' influence on education. She joined the staff in 2013 to write about the intersection of education and business in the pre-K-12 marketplace.